This invention relates to a thermal cut-off fuse of the type in which a thermal pellet remains in a solid state and, consequently, the electric continuity between lead wires is kept intact while the ambient temperature is below a preset level and, when the ambient temperature rises above the preset level, the solid thermal pellet is melted and, consequently, the electric continuity between the lead wires is broken.
The electric cut-off fuse is an element which is used for the purpose of breaking an electric circuit when the electric current flowing through that circuit increases excessively. In contrast, the thermal cut-off fuse is an element which serves the purpose of breaking an electric circuit when the ambient temperature of the fuse rises excessively. Common household electric appliances which have heating sources such as hair driers, stoves and cookers are invariably fitted with a thermal cut-off fuse as a safeguard against excessive heating. The thermal cut-off fuse is available in various designs, some consisting solely of fusible metal wires and others comprising plugs and cases containing such fusible metal wires. Even fusible metal wires of a specific type vary considerably in their melting temperatures and, therefore, do not provide accurate temperature response. Therefore, when such wires are used in electric appliances having heat sources, the metals must have melting temperatures which are considerably lower than the lowest allowable temperatures prescribed for perfect safety of the appliances. To overcome this inconvenience, there has been developed an improved thermal cut-off fuse which uses a thermal pellet made of resin or fatty oils capable of melting precisely at a specific temperature.
The conventional thermal cut-off fuse has an electro-conductive housing and contains therein the aforementioned thermal pellet capable of retaining its solid state below a prescribed temperature level and a resilient metal ring which, in its original free state, has a size amply accommodated within the housing and, upon being deformed by the pressure resulting from the insertion of the thermal pellet in the housing interior, has parts thereof brought into contact with the inner wall of the housing, with one of the lead wires connected to the housing and the other lead wire to the metal ring. The case of a heat-generating appliance to which electric current is supplied through the medium of such a thermal cut-off fuse will now be cited as an example. The supply of electric current to the appliance proceeds so long as the ambient temperature is below the prescribed temperature level. If some abnormality develops within the appliance and the amount of heat generated by the appliance consequently increases so much as to exceed the prescribed level, then the thermal pellet inside the thermal cut-off fuse begins to melt. Melting of the pellet results in a reduction of the volume thereof so that the solid pellet no longer applies pressure to the metal ring and the metal ring is allowed to resume its original state by virtue of its own resiliency. This means that the parts of the metal ring so far held in contact with the inner wall of the housing separate from the wall to break the electric continuity between the lead wires. Consequently, the supply of electric current to the appliance is discontinued to prevent the appliance from excessive heating. Thermal cut-off fuses of this type are finding widespreading acceptance because of their many advantageous such as compactness, simplicity of structure and sensitive response. They nevertheless have a disadvantage in that the metal ring and the inner wall of the housing are brought into contact at points or along lines of limited area as viewed cross-sectionally so that the contact resistance consequently generated tends to become high. When the contact resistance is high, the portions involved generate heat during the passage of electric current. The volume of heat thus generated increases with increasing flow of current. There is a consequent possibility that the heat will melt the thermal pellet. With the conventional thermal pellet, therefore, the rated response temperatures are always fairly low so much that the thermal cut-off fuses are usable only in electric appliances of low capacities.
An object of this invention is to provide a thermal cut-off fuse having very low electric contact resistance and heat generation thus making it possible to produce thermal cut-off fuses with high response temperatures that safely cut off the supply of electric current precisely at the time that the ambient temperature reaches the prescribed level.